Patients who have certain respiratory disease for different medical reasons or who have to undergo surgery must be provided with artificial respiration. A modern mechanical ventilator apparatus for a patient should be adapted to provide sufficient oxygenation and sufficient ventilation for the patient's lung.
In the recent years, a ventilation apparatus has been developed to support the patient respiration and to improve the patients comfort. Therefore, a variety of ventilation modes have been developed that can potentially avoid complications and thus shorten the duration of medical ventilations. One example is the pressure controlled ventilation (PCV), where the ventilator delivers a flow to maintain a set pressure at a present respiratory rate over a present inspiratory time.
Mammals, in particular humans, need a minimum amount of gas exchange in their lung to breath without any technical support, which mainly depends on the alveolar minute volume (Mva). The alveolar minute volume (Mva) is the alveolar volume of gas inhaled or exhaled from a human per minute and is calculated by a respiratory frequency (f) and an alveolar tidal volume (Vta). In 1950 Otis et. al described the mechanical work, done by the human respiratory muscles to produce the movements of breathing, follows the minimum work of breathing triggered by the respiratory muscles. Concerning the assumption that humans breathe spontaneously, without any technical support, Otis et. al found the following correlation between a respiratory frequency (f) and the alveolar minute volume (Mva):
                              f          =                                                                      1                  +                                      4                    *                                          π                      2                                        *                    R                    *                    C                    *                                          Mva                      Vd                                                                                  -              1                                      2              *                              π                2                            *              R              *              C                                      ,                            (                  Eq          .                                          ⁢          1                )            where R is the airway resistance and C is the lung compliance and Vd is the functional dead volume. This correlation is the fundament of one of the mostly used modern ventilation mode for mechanical ventilation—the adaptive support ventilation (ASV) mode. The adaptive support ventilation (ASV) mode was developed by F. Tehrani, who adapted the equation of Otis et. al and modified it with the following substitution,Mva=Mvp−Vd* f,  (Eq. 2)where Mvp is the proximal minute volume, Vd is the functional dead space and f is the respiratory frequency. The resulting respiratory frequency (f) in the ASV mode represents the frequency with which a human being would breathe naturally.
U.S. Pat. No. 4,986,268 A shows a method and an apparatus for automatically controlling a respirator. A software algorithm is used to compute the amount and optimum frequency of ventilation required for the next required breath of the patient. Said frequency of ventilation is calculated based on the minimum work of breathing of the patient and controls said apparatus.
U.S. Pat. No. 8,695,593 B2 shows a method of providing treatment advice for patients on mechanical ventilation. It provides advice on how much ventilation a patient requires and recommends ventilator settings to minimize the respiration work rate based on the ASV mode.
However, the previously described techniques were developed for a specific group of patients. The ASV mode, based on Otis et. al, uses a spontaneously breathing and a non-supported patient as reference. That is, particularly for mandatory ventilation, wrong and for pressure supported ventilation not exactly the same. The pressure course of mandatory breaths as well as pressure supported breaths can be approximated by a rectangular function, while the pressure course of non-supported breaths is more equal to a sine curve. This mismatch results into an increased stress for the patient's lung. Especially, patients with restrictive lung disease, like acute respiratory distress syndrome (ARDS) or Fibrosis must be ventilated with a more protective ventilation mode. Insufficient assistance includes diaphragmatic fatigue or weakness and force the recruitment of accessory inspiration muscles, sometimes leading to respiratory acidosis.